Transplantation is a life-saving procedure for patients with end-stage organ failure. Current immuno- suppression is associated with significant morbidity and a high incidence of chronic transplant dysfunction, rendering the induction of antigen-specific tolerance a desirable alternative. Therapeutic manipulation of the TIM molecules is thought to have great tolerogenic potential. In contrast to the other TIM family members, TIM- 4 is primarily expressed on APCs, including CD11c+ DCs and macrophages. Although in vitro studies utilizing TIM-4 Ig fusion proteins demonstrate that it binds costimulator ligands on T cells, the results are conflicting and the nature of costimulatory signals not well defined. Furthermore, as these co-cultures lacked APCs, the physiological role of TIM-4 on DCs was not examined. TIM-4 has also recently been identified as a phosphatidylserine receptor, capable of modulating the immune system by directing engulfment of apoptotic bodies. The relative contribution of these roles in directing immune responses remains unclear. The primary objective of this application is to address the role of TIM-4 in alloimmunity. We show that antibody-mediated blockade of TIM-4 induces long-term islet allograft survival, suggesting that TIM-4 plays a major role in regulating alloimmunity in vivo. We have also discovered that TIM-4 defines functionally distinct DC subsets involved in the determination of T helper cell fate. Furthermore, TIM-4 blockade inhibits both TIM-4 signaling initiated by TIM-4+ DCs and phagocytosis mediated by macrophages, resulting in the splenic accumulation of apoptotic bodies and a shift of phagocytic duty from macrophages to DCs. The relative contribution of these functions to the marked effect of anti-TIM-4 on graft survival is unknown and is the central focus of this proposal. We hypothesize that the graft prolongation achieved by anti-TIM-4 is due to inhibition of TIM-4 signaling to DCs and/or inhibition of T cell costimulation. Alternatively, TM-4 blockade may alter the activation status of DCs/macrophages and, consequently, allograft survival, by shifting the responsibility of apoptotic cell phagocytosis from macrophages to DCs. Thus, the specific aims are: 1. What is the role of TIM-4 in the regulation of DC phenotype and function? 2. What is the role of TIM-4 on DCs versus macrophages in allograft survival? 3. What are the mechanisms whereby TIM-4 regulates the fate of alloreactive T cells in vivo? These studies will not only provide brand new basic insight into the immunobiology of DCs and Th differentiation, but will allow us to optimize strategies for promoting allograft survival, by selectively inhibiting TIM-4 function on DCs and/or macrophages. Moreover, the insights gained from the proposed studies will have broad therapeutic potential, including the development of tolerogenic strategies in autoimmunity and allergy, while manipulation of this molecule could also be used to enhance the immune response, which would be useful in states of relative immunodeficiency (cancer and chronic infection) and in vaccine development.